The Foundations of Chemistry

UNITS OF MEASUREMENT

Mass and Weight

We distinguish between mass and weight. Massis the measure of the quantity of matter

a body contains (see Section 1-1). The mass of a body does not vary as its position changes.

On the other hand, the weightof a body is a measure of the gravitational attraction of

the earth for the body, and this varies with distance from the center of the earth. An ob-

ject weighs very slightly less high up on a mountain than at the bottom of a deep valley.

Because the mass of a body does not vary with its position, the mass of a body is a more

fundamental property than its weight. We have become accustomed, however, to using

the term “weight” when we mean mass, because weighing is one way of measuring mass

(Figure 1-11). Because we usually discuss chemical reactions at constant gravity, weight

relationships are just as valid as mass relationships. Nevertheless, we should keep in mind

that the two are not identical.

The basic unit of mass in the SI system is the kilogram(Table 1-6). The kilogram is

defined as the mass of a platinum–iridium cylinder stored in a vault in Sèvres, near Paris,

France. A 1-lb object has a mass of 0.4536 kg. The basic mass unit in the earlier metric

systemwas the gram. A U.S. five-cent coin (a “nickel”) has a mass of about 5 g.

Length

The meteris the standard unit of length (distance) in both SI and metric systems. The

meter is defined as the distance light travels in a vacuum in 1/299,792,468 second. It is

approximately 39.37 inches. In situations in which the English system would use inches,

the metric centimeter (1/100 meter) is convenient. The relationship between inches and

centimeters is shown in Figure 1-12.

1-

The meter was originally defined

(1791) as one ten-millionth of the

distance between the North Pole

and the equator.

18 CHAPTER 1: The Foundations of Chemistry

Figure 1-11 Three types of laboratory balances. (a) A triple-beam balance used for

determining mass to about0.01 g. (b) A modern electronic top-loading balance that gives

a direct readout of mass to0.001 g. (c) A modern analytical balance that can be used to

determine mass to0.0001 g. Analytical balances are used when masses must be determined

as precisely as possible.

TABLE 1-6 Some SI Units

of Mass

kilogram, kg base unit

gram, g 1,000 g
1 kg

milligram, mg 1,000 mg
1 g

microgram, g 1,000,000 g
1 g

(a) (b)

(c)